Minimizing the Impact of Self Synchronization on Wireless Communication Devices

ABSTRACT

A method for self synchronization by a base station is described. Network information is sent to a wireless communication device. The network information indicates a first time period. The first time period is a period of silence by the base station. Synchronization signals are monitored during the first time period. Monitoring synchronization signals includes not transmitting.

RELATED APPLICATIONS

The present application for Patent is a Divisional of patent applicationSer. No. 12/755,288 entitled “MINIMIZING THE IMPACT OF SELFSYNCHRONIZATION ON WIRELESS COMMUNICATION DEVICES” filed Apr. 6, 2010,pending, which claims priority from U.S. Provisional Patent ApplicationSer. No. 61/167,653, entitled “METHOD AND APPARATUS FOR MINIMIZING USEREQUIPMENT IMPACT WITH SELF SYNCHRONIZATION” filed Apr. 8, 2009.

TECHNICAL FIELD

The present disclosure relates generally to wireless communicationsystems. More specifically, the present disclosure relates to systemsand methods for minimizing the impact of self synchronization onwireless communication devices.

BACKGROUND

Wireless communication systems have become an important means by whichmany people worldwide have come to communicate. A wireless communicationsystem may provide communication for a number of mobile stations, eachof which may be serviced by a base station.

Each of the base stations in a wireless communication system may operatesynchronously. In other words, each of the base stations may synchronizeclocks with the same source. By operating synchronously, improvementssuch as interference management may be achieved.

In addition to the wireless communication systems currently in place, anew class of small base stations has emerged. These small base stationsmay be installed in a user's home and provide indoor wireless coverageto mobile stations using existing broadband Internet connections.Typically, these miniature base stations are connected to the Internetand the mobile device's network via a Digital Subscriber Line (DSL)router or cable modem. Benefits may be realized by improved methods forsynchronizing these miniature base stations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system with multiple wirelessdevices;

FIG. 2 is a flow diagram of a method for self synchronization of a basestation;

FIG. 3 illustrates the transmission of network information that includesa multimedia broadcast single frequency network (MBSFN) declaration froma home evolved NodeB (HeNB) to a wireless communication device;

FIG. 4 illustrates the transmission of network information that includesa discontinuous receive (DRX) mode message from a home evolved NodeB(HeNB) to a wireless communication device;

FIG. 5 illustrates the transmission of network information that includesscheduling information from a home evolved NodeB (HeNB) to a wirelesscommunication device;

FIG. 6 illustrates the transmission of network information that includesa subframe index from a home evolved NodeB (HeNB) to a wirelesscommunication device;

FIG. 7 is a timing diagram illustrating self synchronization withmultiple coordinated silence periods;

FIG. 8 is a flow diagram of a method for self synchronization usingmultimedia broadcast single frequency network (MBSFN) subframes;

FIG. 9 is a timing diagram illustrating tracking using multimediabroadcast single frequency network (MBSFN) subframes;

FIG. 10 is a flow diagram of another method for self synchronizationusing multimedia broadcast single frequency network (MBSFN) subframes;

FIG. 11 is a flow diagram of a method for receiving self synchronizationnetwork information;

FIG. 12 shows a wireless communication system with multiple wirelessdevices and their respective stratum;

FIG. 13 illustrates certain components that may be included within abase station; and

FIG. 14 illustrates certain components that may be included within awireless communication device.

DETAILED DESCRIPTION

A method for self synchronization by a base station is described.Network information is sent to a wireless communication device. Thenetwork information indicates a first time period that is a period ofsilence by the base station. Synchronization signals are monitoredduring the first time period. Monitoring synchronization signalsincludes not transmitting.

The base station may have a current stratum. Synchronization signals maybe transmitted in a second time period that includes slots designatedfor base stations having a stratum less than or equal to the currentstratum to transmit synchronization signals. The first time period mayinclude slots designated for base stations having a stratum less than orequal to the current stratum to monitor synchronization signals. Thenetwork information may include a multimedia broadcast single frequencynetwork (MBSFN) subframe declaration. A common reference signal (CRS)may be received from a synchronizing node. The base station may use theCRS for tracking.

A common reference signal (CRS) may be transmitted in subframes declaredas MBSFN for a stratum greater the current stratum. The subframesdeclared as MBSFN may be part of a second time period. A commonreference signal (CRS) may be tracked in subframes declared as MBSFN fora stratum less than or equal to the current stratum.

A synchronization signal may be searched for. It may be determinedwhether the synchronization signal has been acquired. A current stratummay be determined based on the synchronization signal if thesynchronization signal has been acquired. The network information mayinclude a message instructing the one or more wireless communicationdevices to enter a discontinuous receive (DRX) mode during the period ofsilence. The network information may also include a sleep time for thewireless communication device. The network information may identify oneor more subframes when the base station performs self synchronization.

The network information may include a subframe index that implicitlyindicates one or more subframes when the base station performs selfsynchronization. The synchronization signals may be sent by asynchronization source. Monitoring synchronization signals may includetime-tracking a synchronization source. Frequency error correction maybe performed while time-tracking the synchronization source.

Sending the network information may include gradually decreasing powerand then gradually increasing the power to mimic a deep fade. Thenetwork information may include scheduling information that includes noscheduled transmissions to the wireless communication device in andaround subframes where no common reference signal (CRS) is transmitted.Sending the network information may also include simultaneouslytransmitting a common reference signal (CRS) to the wirelesscommunication device while achieving self synchronization using aPrimary Synchronization Signal (PSS) on the last two orthogonalfrequency division multiplexing (OFDM) symbols of a slot.

Sending the network information may further include simultaneouslytransmitting a common reference signal (CRS) to the wirelesscommunication device while achieving self synchronization using aSecondary Synchronization Signal (SSS) on the last two orthogonalfrequency division multiplexing (OFDM) symbols of a slot.

The base station may be a home evolved NodeB (HeNB). The wirelesscommunication device may be legacy user equipment (UE). The period ofsilence may be coordinated by a network.

A wireless device configured for self synchronization is described. Thewireless device includes a processor, memory in electronic communicationwith the processor, and instructions stored in the memory. Theinstructions are executable by the processor to send network informationto a wireless communication device. The network information indicates afirst time period that is a period of silence by the base station. Theinstructions are also executable by the processor to monitorsynchronization signals during the first time period. Monitoringsynchronization signals includes not transmitting.

A wireless device configured for self synchronization is also described.The wireless device includes means for sending network information to awireless communication device. The network information indicates a firsttime period that is a period of silence by the base station. Thewireless device also includes means for monitoring synchronizationsignals during the first time period. Monitoring synchronization signalsincludes not transmitting.

A computer-program product for self synchronization by a base station isdescribed. The computer-program product includes a computer-readablemedium having instructions thereon. The instructions include code forsending network information to a wireless communication device. Thenetwork information indicates a first time period that is a period ofsilence by the base station. The instructions also include code formonitoring synchronization signals during the first time period.Monitoring synchronization signals includes not transmitting.

The 3^(rd) Generation Partnership Project (3GPP) is a collaborationbetween groups of telecommunications associations that aims to define aglobally applicable third generation (3G) mobile phone specification.3GPP Long Term Evolution (LTE) is a 3GPP project aimed at improving theUniversal Mobile Telecommunications System (UMTS) mobile phone standard.The 3GPP may define specifications for the next generation of mobilenetworks, mobile systems, and mobile devices.

In 3GPP LTE, a mobile station or device may be referred to as a “userequipment” (UE). A base station may be referred to as an evolved NodeB(eNB). A semi-autonomous base station may be referred to as a home eNB(HeNB). An HeNB may thus be one example of an eNB. The HeNB and/or thecoverage area of an HeNB may be referred to as a femtocell, a picocell,an HeNB cell or a closed subscriber group (CSG) cell.

FIG. 1 shows a wireless communication system 100 with multiple wirelessdevices. Wireless communication systems 100 are widely deployed toprovide various types of communication content such as voice, data, andso on. These systems may be multiple-access systems capable ofsupporting communication with multiple users by sharing the availablesystem resources (e.g., bandwidth and transmit power). A wireless devicemay be a base station 104 or a wireless communication device 114. Aglobal positioning system (GPS) server 102 is also illustrated in FIG.1.

A base station 104 is a station that communicates with one or morewireless communication devices 114. A base station 104 may also bereferred to as, and may include some or all of the functionality of, anaccess point, a broadcast transmitter, a NodeB, an evolved NodeB, etc.The term “Base Station” will be used herein. Each base station 104provides communication coverage for a particular geographic area. A basestation 104 may provide communication coverage for one or more wirelesscommunication devices 114. The term “cell” can refer to a base station104 and/or its coverage area depending on the context in which the termis used.

Communications in a wireless system (e.g., a multiple-access system) maybe achieved through transmissions over a wireless link. Such acommunication link may be established via a single-input andsingle-output (SISO), multiple-input and single-output (MISO), or amultiple-input and multiple-output (MIMO) system. A MIMO system includestransmitter(s) and receiver(s) equipped, respectively, with multiple(NT) transmit antennas and multiple (NR) receive antennas for datatransmission. SISO and MISO systems are particular instances of a MIMOsystem. The MIMO system can provide improved performance (e.g., higherthroughput, greater capacity, or improved reliability) if the additionaldimensionalities created by the multiple transmit and receive antennasare utilized.

The wireless communication system 100 may utilize MIMO. A MIMO systemmay support both time division duplex (TDD) and frequency divisionduplex (FDD) systems. In a TDD system, forward and reverse linktransmissions are on the same frequency region so that the reciprocityprinciple allows the estimation of the forward link channel from thereverse link channel. This enables a transmitting wireless device toextract transmit beamforming gain from communications received by thetransmitting wireless device.

The wireless communication system 100 may be a multiple-access systemcapable of supporting communication with multiple wireless communicationdevices by sharing the available system resources (e.g., bandwidth andtransmit power). Examples of such multiple-access systems include codedivision multiple access (CDMA) systems, wideband code division multipleaccess (W-CDMA) systems, time division multiple access (TDMA) systems,frequency division multiple access (FDMA) systems, orthogonal frequencydivision multiple access (OFDMA) systems, single-carrier frequencydivision multiple access (SC-FDMA) systems, Third Generation PartnershipProject (3GPP) Long Term Evolution (LTE) systems, and spatial divisionmultiple access (SDMA) systems.

The terms “networks” and “systems” are often used interchangeably. ACDMA network may implement a radio technology such as UniversalTerrestrial Radio Access (UTRA), cdma2000, etc. UTRA includes W-CDMA andLow Chip Rate (LCR) while cdma2000 covers IS-2000, IS-95 and IS-856standards. A TDMA network may implement a radio technology such asGlobal System for Mobile Communications (GSM). An OFDMA network mayimplement a radio technology such as Evolved UTRA (E-UTRA), IEEE 802.11,IEEE 802.16, IEEE 802.20, Flash-OFDMA, etc. UTRA, E-UTRA, and GSM arepart of Universal Mobile Telecommunication System (UMTS). Long TermEvolution (LTE) is a release of UMTS that uses E-UTRA. UTRA, E-UTRA,GSM, UMTS and LTE are described in documents from an organization named“3rd Generation Partnership Project” (3GPP). cdma2000 is described indocuments from an organization named “3rd Generation Partnership Project2” (3GPP2). For clarity, certain aspects of the techniques are describedbelow for LTE, and LTE terminology is used in much of the descriptionbelow.

Single carrier frequency division multiple access (SC-FDMA) systemsutilize single carrier modulation and frequency domain equalization. AnSC-FDMA system has similar performance and essentially the same overallcomplexity as those of an OFDMA system. An SC-FDMA signal has lowerpeak-to-average power ratio (PAPR) because of its inherent singlecarrier structure. SC-FDMA has drawn great attention, especially inuplink communications where lower peak to average power ratio (PAPR)greatly benefits the mobile terminal in terms of transmit powerefficiency. It is currently a working assumption for uplink multipleaccess scheme in 3GPP Long Term Evolution (LTE), or Evolved UTRA.

Synchronization among base stations 104 in a wireless network bringsmany benefits such as interference management or virtual MIMO.Traditionally, cellular network synchronization is achieved using globalpositioning system (GPS) receivers collocated with base stations 104.GPS receivers and/or GPS signals may not always be available forsynchronization purposes due to manufacturing cost consideration, powerconsumption limitations, the lack of line-of-sight to GPS satellites andother reasons. In such scenarios, alternative synchronization strategiesmay be necessary. One such scenario is the heterogeneous deploymentsused in LTE or LTE-A.

Less power base stations 104 such as home evolved NodeBs (HeNB),picocells and femtocells are used in addition to the normal basestations 104. A picocell may refer to a base station 104 controlled bythe network operator that operates on a much smaller scale than normalbase stations 104. A femtocell may refer to a base station 104controlled by a consumer that operates on a much smaller scale thannormal base stations 104. A femtocell may provide service to a closedsubscriber group. A femtocell, picocell and HeNB may have similartransmit powers and coverage areas. A femtocell, picocell and HeNB maybe placed indoors where they are unlikely to receive a GPS signal.Alternatively, a femtocell, picocell or HeNB may not even have a GPSreceiver. A normal base station 104 may be referred to as a macro basestation 104.

An unsynchronized base station 104 may derive synchronization from analready synchronized base station 104. Once a base station 104 hasderived synchronization from an already synchronized base station 104,the newly synchronized base station 104 may continue to monitorsynchronization signals transmitted by the already synchronized basestation 104. For example, the first base station 104 a may have derivedsynchronization from the second base station 104 b. The first basestation 104 a may continue to monitor synchronization signals (transmit)110 b that are transmitted by the second base station 104 b. Thesynchronization signals received by the first base station 104 a fromthe second base station 104 b may be referred to as synchronizationsignals (received) 110 a. The first base station 104 a may use thesynchronization signals (received) 110 a to derive synchronization.

The second base station 104 b may be synchronized directly with a globalpositioning system (GPS) server 102. The second base station 104 b maybe referred to as a macro evolved NodeB. The second base station 104 bmay use a GPS receiver (not shown) to synchronize directly with theglobal positioning system (GPS) server 102.

The first base station 104 a may also provide synchronization to a thirdbase station 104 b. The first base station 104 a may transmitsynchronization signals (transmit) 108 a. Synchronization signalsreceived by the third base station 104 c from the first base station 104a may be referred to as synchronization signals (received) 108 b. Thethird base station 104 c may use the synchronization signals (received)108 b to derive synchronization.

The first base station 104 a may communicate with one or more wirelesscommunication devices 114. A wireless communication device 114 may alsobe referred to as, and may include some or all of the functionality of,a terminal, an access terminal, a user equipment (UE), a subscriberunit, a station, etc. A wireless communication device 114 may be acellular phone, a personal digital assistant (PDA), a wireless device, awireless modem, a handheld device, a laptop computer, etc. A wirelesscommunication device 114 may communicate with zero, one, or multiplebase stations 104 on the downlink 116 and/or uplink 118 at any givenmoment. The downlink 116 (or forward link) refers to the communicationlink from a base station 104 to a wireless communication device 114, andthe uplink 118 (or reverse link) refers to the communication link from awireless communication device 114 to a base station 104. As part ofcommunicating with a wireless communication device 114, the first basestation 104 a may transmit a common reference signal (CRS) 120 to thewireless communication device 114. The common reference signal (CRS) 120may be used by the wireless communication device 114 for cell search,initial acquisition and downlink 116 channel quality measurements.

The first base station 104 a may include a current number 106 ofintermediate synchronous nodes between the base station and the globalpositioning system (GPS) server. The number 106 of intermediatesynchronous nodes between a base station 104 and the global positionsystem (GPS) server 102 may be referred to as stratum. In FIG. 1, thecurrent number 106 of intermediate synchronous nodes between the firstbase station 104 a and the global positioning system (GPS) server 102 isone (the second base station 104 b). Thus, the first base station 104 ahas a current stratum of Stratum-1.

The first base station 104 a may include a coordinated silence module112. As discussed above, the first base station 104 a may acquiresynchronization from the second base station 104 b and providesynchronization to the third base station 104 b. When the first basestation 104 a is acquiring synchronization from the second base station104 b, the first base station 104 a may monitor the synchronizationsignals (transmit) 110 b from the second base station 104 b. To monitorthe synchronization signals (transmit) 110 b from the second basestation 104 b, the first base station 104 a may have to stoptransmitting to the wireless communication device 114 in some slotswhere the first base station 104 a may normally transmit. Thesetransmissions may include the common reference signal (CRS) 120 as wellas other network information or data transfers.

These periods of silence may impact a wireless communication device 114that is expecting transmissions from the first base station 104 a and/ortrying to perform channel measurements. Even if the wirelesscommunication device 114 is not scheduled in some subframes, thewireless communication device 114 may still expect the transmission ofthe common reference signal (CRS) 120 from the first base station 104 a.If the first base station 104 a is monitoring the synchronizationsignals (transmit) 110 b of the second base station 104 b ortransmitting synchronization signals (transmit) 108 a to the third basestation 104 c, the first base station 104 a may not transmit the commonreference signal (CRS) 120 to the wireless communication device 114. Inorder to not disrupt the wireless communication device 114 and the wholewireless communication system 100, the impact of these coordinatedsilence periods needs to be minimized.

The coordinated silence module 112 may determine the length and startingtimes of each coordinated silence period. The length and starting timefor each coordinated silence period may be controlled by the wirelesscommunication network 100. The coordinated silence module 112 mayminimize the impact of coordinated silence periods on a wirelesscommunication device 114.

Different methods may be used to support self synchronization whileminimizing the impact on the wireless communication device 114. Forexample, the coordinated silence module 112 may inform the wirelesscommunication device 114 of upcoming coordinated silence periods. Asanother example, the coordinated silence module 112 may use a channelsuch as the Primary Synchronization Signal (PSS) 109 a or SecondarySynchronization Signal (SSS) 109 b to achieve synchronization with thesecond base station 104 b while continuing to transmit the commonreference signal (CRS) 120. The PSS/SSS 109 occupy only the last twoOFDM symbols of a slot and do not overlap with the common referencesignal (CRS) 120 in either the time domain or the frequency domain.Thus, the common reference signal (CRS) 120 can still be transmittedwhile the first base station 104 a is monitoring the PSS/SSS 109 of thesecond base station 104 b. Tracking the PSS/SSS 109 may come at the costof some backwards compatibility since the first base station 104 a wouldneed to shut down its PSS/SSS transmission to monitor the PSS/SSS 109 ofthe donor second base station 104 b. In yet another example, thecoordinated silence module 112 may gradually lower and then increasepower to the wireless communication device 114 to mimic a fade. Thewireless communication device 114 may interpret the absence of a commonreference signal (CRS) 120 to be caused by the propagation environmentand wait for conditions to improve. This would depend on theimplantation and duration of the fade.

FIG. 2 is a flow diagram of a method 200 for self synchronization of abase station 104. The method 200 may be performed by the first basestation 104 a. As discussed above, the first base station 104 a may be ahome evolved NodeB (HeNB). The first base station 104 a may determine202 a current number 106 of intermediate synchronous nodes between thefirst base station 104 a and a global positioning system (GPS) server102. The number 106 of intermediate synchronous nodes between a basestation 104 and the global positioning system (GPS) server 102 may bereferred to as the stratum. The first base station 104 a may thendetermine 204 a first coordinated silence period and a secondcoordinated silence period based on the current number 106 ofintermediate synchronous nodes between the first base station 104 a andthe global positioning system (GPS) server 102. Additional coordinatedsilence periods may also be determined.

The first base station 104 a may send 206 network information to awireless communication device 114. The network information may be basedon the determined first coordinated silence period and the determinedsecond coordinated silence period. The network information may beinformation designed to minimize the impact of the coordinated silenceperiods on the wireless communication device 114.

The network information may include a multimedia broadcast singlefrequency network (MBSFN) subframe declaration. In MBSFN, a transmissionhappens from a time-synchronized set of evolved NodeBs using the sameresource block. The use of MBSFN may improve the signal to interferenceplus noise ratio (SINR) by enabling over the air combining. If asubframe is declared to be MBSFN, a wireless communication device 114will ignore the later part of this subframe. MBSFN subframe declarationsare discussed in additional detail below in relation to FIG. 3.

The network information may include a discontinuous receive (DRX) modemessage. A DRX mode message may cause a wireless communication device114 to sleep for a longer period of time. This longer period of time maycoincide with the first coordinated silence period and/or the secondcoordinated silence period. The wireless communication device 114 maythen not be affected by the lack of transmissions from the first basestation 104 a. DRX mode messages are discussed in further detail belowin relation to FIG. 4.

The network information may include scheduling information. Thescheduling information may indicate to the wireless communication device114 when transmissions are scheduled from the first base station 104 ato the wireless communication device 114. By not scheduling anytransmissions to the wireless communication device 114 in and around thesubframes where no common reference signal (CRS) 120 is transmitted, theimpact of using coordinated silence periods may be minimized Schedulinginformation is discussed in additional detail below in relation to FIG.5.

The network information may include a subframe index. The subframe indexmay inform the wireless communication device 114 of subframes that thefirst base station 104 a will not transmit in. If a wirelesscommunication device 114 is aware of the subframes in which the firstbase station 104 a is performing self synchronization, the wirelesscommunication device 114 may avoid using those subframes for commonreference signal (CRS) 120 estimation. Subframe indexes are discussed infurther detail below in relation to FIG. 6.

After the first base station 104 a has sent the network information tothe wireless communication device 114, the first base station 104 a maymonitor 208 synchronization signals (transmit) 110 b during the firstcoordinated silence period. The synchronization signals (transmit) 110 bmay be transmitted by a synchronizing base station 104 such as thesecond base station 104 b. The first base station 104 a may ceasetransmissions while monitoring 208 the synchronization signals(transmit) 110 b. In one configuration, the first base station 104 a maycontinue transmitting the common reference signal (CRS) 120 whilemonitoring the synchronization signals (transmit) 110 b.

The first base station 104 a may transmit 210 synchronization signals(transmit) 108 a during the second coordinated silence period. Thesynchronization signals (transmit) 108 a may be broadcast so that allnearby base stations 104 can receive them. The first base station 104 amay not send any transmissions to the wireless communication device 114during the second coordinated silence period. In one configuration, thefirst base station 104 a may continue transmitting the common referencesignal (CRS) 120 while transmitting the synchronization signals(transmit) 108 a. The first base station 104 a may perform 212 frequencyerror correction. In one configuration, the first base station 104 a mayperform 212 frequency error correction while time-tracking asynchronization source (i.e., receiving synchronization signals(transmit) 110 b from the second base station 104 b). The first basestation 104 a may also perform timing error correction.

FIG. 3 illustrates the transmission of network information 311 thatincludes a multimedia broadcast single frequency network (MBSFN)declaration 322 from a home evolved NodeB (HeNB) 304 to a wirelesscommunication device 314. The home evolved NodeB (HeNB) 304 of FIG. 3may be one configuration of the first base station 104 a of FIG. 1. Thewireless communication device 314 of FIG. 3 may be one configuration ofthe wireless communication device 114 of FIG. 1.

The network information 311 may include a multimedia broadcast singlefrequency network (MBSFN) subframe declaration 322. The multimediabroadcast single frequency network (MBSFN) subframe declaration 322 mayinclude a bitmap indicating the multimedia broadcast single frequencynetwork (MBSFN) subframe in one of the system information messages(e.g., a system information block (SIB)). As discussed above, parts ofsubframes declared as multimedia broadcast single frequency network(MBSFN) subframes may be ignored by the wireless communication device314. The home evolved NodeB (HeNB) 304 can use the common referencesignal (CRS) 120 transmitted by the synchronizing base station 104(i.e., the second base station 104 b) for tracking. The synchronizingbase station 104 does not need to declare a multimedia broadcast singlefrequency network (MBSFN) subframe.

FIG. 4 illustrates the transmission of network information 411 thatincludes a discontinuous receive (DRX) mode message 424 from a homeevolved NodeB (HeNB) 404 to a wireless communication device 414. Thehome evolved NodeB (HeNB) 404 of FIG. 4 may be one configuration of thefirst base station 104 a of FIG. 1. The wireless communication device414 of FIG. 4 may be one configuration of the wireless communicationdevice 114 of FIG. 1. The discontinuous receive (DRX) mode message 424may instruct the wireless communication device 414 to enter adiscontinuous receive (DRX) mode, where the wireless communicationdevice 414 sleeps for an extended period of time. If the wirelesscommunication device 414 is asleep when the home evolved NodeB (HeNB)404 performs self synchronization (i.e., during the first coordinatedsilence period and the second coordinated silence period), the selfsynchronization may not affect the performance of the wirelesscommunication device 414. A wireless communication device 414 may beconfigured for discontinuous receive (DRX) mode by upper layers (such asthe Radio Resource Control (RRC)). The discontinuous receive (DRX) modemessage 424 may include the amount of time that the wirelesscommunication device 414 sleeps (sleep time 470) and the time thewireless communication device 414 searches for the control channel(search time 471).

FIG. 5 illustrates the transmission of network information 511 thatincludes scheduling information 525 from a home evolved NodeB (HeNB) 504to a wireless communication device 514. The home evolved NodeB (HeNB)504 of FIG. 5 may be one configuration of the first base station 104 aof FIG. 1. The wireless communication device 514 of FIG. 5 may be oneconfiguration of the wireless communication device 114 of FIG. 1. Thescheduling information 525 may implicitly inform the wirelesscommunication device 514 of the coordinated silence periods by notscheduling any transmissions to the wireless communication device 514during the coordinated silence periods.

FIG. 6 illustrates the transmission of network information 611 thatincludes a subframe index 626 from a home evolved NodeB (HeNB) 604 to awireless communication device 614. The home evolved NodeB (HeNB) 604 ofFIG. 6 may be one configuration of the first base station 104 a ofFIG. 1. The wireless communication device 614 of FIG. 6 may be oneconfiguration of the wireless communication device 114 of FIG. 1. Asdiscussed above, the subframe index 626 may explicitly inform thewireless communication device 614 of subframes that the home evolvedNodeB (HeNB) 604 will not transmit in. The effect of selfsynchronization by the home evolved NodeB (HeNB) 604 may be minimized ifthe wireless communication device 614 is aware of the subframes when notransmissions from the home evolved NodeB (HeNB) 604 will be received.The subframe index 626 may also indicate that the subframe is not usedfor downlink 116 transmissions. The sending of a subframe index 626 tothe wireless communication device 614 may not be supported if thewireless communication device 614 is a legacy device but could besupported in future releases. The subframes may be indicated explicitlyor implicitly (e.g., as a function of the subframe index 626).

FIG. 7 is a timing diagram illustrating self synchronization withmultiple coordinated silence periods 730. Each coordinated silenceperiod 730 may correspond to the self synchronization of one or morebase stations 704. For example, a first coordinate silence period 730 amay correspond to the self synchronization of a first base station 704 aand a second base station 704 b. The first base station 704 a may have astratum of Stratum-1. The second base station 704 b may have a stratumof Stratum-0. The first base station 704 a may derive synchronizationfrom the second base station 704 b. The second base station 704 b mayderive synchronization from a global positioning system (GPS) source102.

During the first coordinated silence period 730 a, the first basestation 704 a may monitor 734 synchronization signals and the secondbase station 704 b may transmit 732 a synchronization signals. The firstbase station 704 a may monitor 734 synchronization signals from multiplebase stations 104 including the second base station 704 b. Thesynchronization signals may be synchronization signals (transmit) 110 bsuch as the Primary Synchronization Signal (PSS) 109 a and the SecondarySynchronization Signal (SSS) 109 b. The second base station 704 b maytransmit 732 a synchronization signals to other base stations 104 inaddition to the first base station 704 a. A third base station 704 c mayderive synchronization from the first base station 704 a. The third basestation 704 c may have a stratum of Stratum-2. During the firstcoordinated silence period 730 a, the third base station 704 c mayremain silent while attempting to monitor 738 a for synchronizationsignals. If the third base station 704 c detects a change in thenetwork, the third base station 704 c may be able to modify the stratumaccordingly.

A second coordinated silence period 730 b may correspond to the selfsynchronization of the first base station 704 a, the second base station704 b and the third base station 704 c. During the second coordinatedsilence period 730 b, the first base station 704 a may transmit 736 asynchronization signals such synchronization signals (transmit) 108 a.The first base station 704 a may transmit 736 a synchronization signalsto multiple base stations 104, including the third base station 704 c.During the second coordinated silence period 730 b, the third basestation 704 c may monitor 738 b synchronization signals. The second basestation 704 b may also transmit 732 b synchronization signals during thesecond coordinated silence period 730 b.

A third coordinated silence period 730 c may correspond to the selfsynchronization of the first base station 704 a, the second base station704 b and the third base station 704 c. During the third coordinatedsilence period 730 c, the third base station 704 c may transmit 740synchronization signals. The first base station 704 a may also transmit736 b synchronization signals during the third coordinated silenceperiod 730 c. In addition, the second base station 704 b may transmit732 c synchronization signals during the third coordinated silenceperiod 730 c. Other base stations 104 (not shown) may receive thesynchronization signals transmitted by the third base station 704 c.

FIG. 8 is a flow diagram of a method 800 for self synchronization usingmultimedia broadcast single frequency network (MBSFN) subframes. Themethod 800 of FIG. 8 may be performed by a first base station 104 a. Thefirst base station 104 a may be a home evolved NodeB (HeNB) 304. Thefirst base station 104 a may send 802 network information 311 to awireless communication device 114 declaring a subframe as a multimediabroadcast single frequency network (MBSFN) subframe.

The first base station 104 a may then monitor 804 the common referencesignal (CRS) of a synchronizing base station 104 b during a first partof the multimedia broadcast single frequency network (MBSFN) subframe.The first base station 104 a may transmit 806 synchronization signals(transmit) 108 a during a second part of the multimedia broadcast singlefrequency network (MBSFN) subframe. A subframe may have two slots (i.e.,each slot may be a part of the subframe). The first part (or first slot)of the multimedia broadcast single frequency network (MBSFN) subframemay be used for monitoring while the second part (or second slot) may beused for transmitting. In one configuration, an entire subframe may beused for either monitoring or transmitting. As discussed above, awireless communication device 114 may ignore subframes that have beendeclared as multimedia broadcast single frequency network (MBSFN)subframes.

FIG. 9 is a timing diagram illustrating tracking using multimediabroadcast single frequency network (MBSFN) subframes. A home evolvedNodeB (HeNB) 314 may stop transmitting for a subframe 948 to tracksynchronization. To minimize the impact of tracking synchronization, thehome evolved NodeB (HeNB) 314 may declare this subframe 948 to be amultimedia broadcast single frequency network (MBSFN) subframe. Thismethod allows for multiple hops in the synchronization path. Also, allthe nodes in a wireless communication system 100 can track in acoordinated fashion (by all declaring multimedia broadcast singlefrequency network (MBSFN) subframes at the same time), thus minimizinginterference.

Timing structures for a macro evolved NodeB 942, a first home evolvedNodeB (HeNB) 944 and a second home evolved NodeB (HeNB) 946 areillustrated. The macro evolved NodeB 942 of FIG. 9 may be oneconfiguration of the second base station 104 b of FIG. 1 and may have astratum of Stratum-0. The first home evolved NodeB (HeNB) 944 of FIG. 9may be one configuration of the first base station 104 a of FIG. 1 andmay have a stratum of Stratum-1. The second home evolved NodeB (HeNB)946 of FIG. 9 may be one configuration of the third base station 104 cof FIG. 1 and may have a stratum of Stratum-2.

During a first subframe 948 a, the macro evolved NodeB 942 may transmit950 a a common reference signal (CRS). The first home evolved NodeB(HeNB) 944 may also transmit 952 a a common reference signal (CRS) 120.Also, the second home evolved NodeB (HeNB) 946 may transmit 956 a commonreference signal (CRS).

The second subframe 948 b of FIG. 9 may be one configuration of thefirst coordinated silence period 730 a of FIG. 7. The macro evolvedNodeB 942 may transmit 950 b the common reference signal (CRS) duringthe second subframe 948 b. The first home evolved NodeB (HeNB) 944 mayreceive 954 synchronization signals during the second subframe 948 b.Thus, the first home evolved NodeB (HeNB) 944 may need to declare thesecond subframe 948 b as a multimedia broadcast single frequency network(MBSFN) subframe to those wireless communication devices 114communicating with the first home evolved NodeB (HeNB) 944. The secondhome evolved NodeB (HeNB) 946 may also receive 958 a synchronizationsignals during the second subframe 948 b. Thus, the second home evolvedNodeB (HeNB) 946 may also need to declare the second subframe 948 b as amultimedia broadcast single frequency network (MBSFN) subframe. In oneconfiguration, the second home evolved NodeB (HeNB) 946 may find andreceive 958 a synchronization signals during the second subframe 948 bfrom another base station 104 with a lower stratum.

The third subframe 948 c of FIG. 9 may be one configuration of thesecond coordinated silence period 730 b of FIG. 7. The macro evolvedNodeB 942 may transmit 950 c the common reference signal (CRS) duringthe third subframe 948 c. The first home evolved NodeB (HeNB) 944 mayalso transmit 952 b the common reference signal (CRS) 120 during thethird subframe 948 c. The second home evolved NodeB (HeNB) 946 mayreceive 958 b synchronization signals during the third subframe 948 cfrom the first home evolved NodeB (HeNB) 944. Therefore, the second homeevolved NodeB (HeNB) 946 may need to declare the third subframe 948 c asa multimedia broadcast single frequency network (MBSFN) subframe tothose wireless communication devices 114 communicating with the secondhome evolved NodeB (HeNB) 946.

Using multimedia broadcast single frame network (MBSFN) subframesensures that the entire network uses the same synchronization source(e.g., a global navigation satellite system (GNSS) such as the globalpositioning system (GPS) server 102) and loops are not created. This isbecause each home evolved NodeB (HeNB) 944, 946 declares its stratum asone greater than that of its donor base station 104. The stratum numberof a home evolved NodeB (HeNB) 944, 946 is self-configured. In addition,the home evolved NodeB (HeNB) 944, 946 tries to track the node with thelowest available stratum. This in turn allows the home evolved NodeB(HeNB) 944, 946 to be as close to the timing of the GNSS as possible.The stratum number is a dynamic quality that could vary with changingconditions (such as home evolved NodeBs (HeNBs) 944, 946 being shutoff).

FIG. 10 is a flow diagram of another method 1000 for selfsynchronization. The method 1000 may be performed by a base station 104a. The base station 104 a may be a home evolved NodeB (HeNB) 304. Thebase station 104 a may first power up 1002. The base station 104 a maythen search 1004 for a synchronization signal (such as a PrimarySynchronization Signal (PSS), a Secondary Synchronization Signal (SSS)or a common reference signal (CRS)). While the base station 104 asearches 1004 for a synchronization signal, transmissions from the basestation 104 may be turned off. The base station 104 a may then determine1006 whether a synchronization signal has been acquired. If asynchronization signal has not been acquired and the base station 104 ais operating in a time division duplexing (TDD) mode, the base station104 a may continue searching 1004 for a synchronization signal. If asynchronization signal has not been acquired and the base station 104 ais operating in a frequency division duplexing (FDD) mode, the basestation 104 a may transmit 1007 until a searching time begins. Once thesearching time begins, the base station 104 a may return to searching1004 for synchronization signals.

If a synchronization signal has been acquired, the base station 104 amay determine 1008 a current stratum. The current stratum may be onegreater than the stratum of a synchronizing base station 104 bbroadcasting the synchronization signal. The stratum of a synchronizingbase station 104 b may be included in the broadcast of thesynchronization signal.

The base station 104 a may transmit 1010 synchronization signals (suchas a common reference signal (CRS) 120) in slots designated for basestations having a stratum less than or equal to the current stratum totransmit synchronization signals. The base station 104 a may also track1012 synchronization signals (such as a common reference signal (CRS)120) in slots designated for base stations having a stratum less than orequal to the current stratum to monitor synchronization signals. Inother words, the base station 104 a may transmit synchronization signals(transmit) 108 a in a first coordinated silence period. During the firstcoordinated silence period, all base stations 104 with a stratum lessthan or equal to the current stratum will also transmit synchronizationsignals. The base station 104 a may remain silent in a secondcoordinated silence period and track synchronization signals (received)110 a. During the second coordinated silence period, one or more basestations 104 with a stratum greater than the current stratum may also betracking synchronization signals.

There may be a period of silence for each stratum. For example, during afirst period of silence, all base stations 104 with a stratum ofStratum-0 may transmit. During the first period of silence, all otherbase stations 104 (i.e., base stations with a stratum of Stratum-1,Stratum-2 . . . ) remain silent while monitoring synchronizationsignals. During a second period of silence, all base stations 104 with astratum of Stratum-1 and Stratum-0 may transmit. During the secondperiod of silence, all other base stations 104 (i.e., base stations witha stratum of Stratum-2, Stratum-3 . . . ) remain silent while monitoringsynchronization signals.

The base station 104 a may then determine 1014 whether synchronizationwas acquired/tracking was successful. If synchronization isacquired/tracking is successful, the base station 104 a may derive 1008a new current stratum. If synchronization is not acquired/tracking isnot successful, the base station 104 a may return to searching 1004 fora synchronization signal.

The overhead incurred by this scheme may depend on the number of hopsbetween a base station 104 a and the global positioning system (GPS)source 102. The overhead may be computed as the number of hopsmultiplied by one subframe in every 320 subframes. The multimediabroadcast single frequency network (MBSFN) subframe method can be usedfor frequency division duplexing (FDD) for deriving frequencysynchronization and potentially time synchronization.

FIG. 11 is a flow diagram of a method 1100 for receiving selfsynchronization network information 311. The method of FIG. 11 may beperformed by a wireless communication device 114. The wirelesscommunication device 114 may receive 1102 network information 311 from ahome evolved NodeB (HeNB) 304. The network information 311 may include amultimedia broadcast single frequency network (MBSFN) subframedeclaration 322, a discontinuous receive (DRX) mode message 424,scheduling information 525, or a subframe index 626.

The wireless communication device 114 may ignore 1104 a later part ofsubframes 948 declared as multimedia broadcast single frequency network(MBSFN) subframes. The wireless communication device 114 may also enter1106 a discontinuous receive (DRX) mode during a coordinated silenceperiod. The wireless communication device 114 may further refrain 1108from using subframes 948 for common reference signal (CRS) estimationthat the home evolved NodeB (HeNB) 304 uses for self synchronization.

FIG. 12 shows a wireless communication system 1200 with multiplewireless devices and their respective stratum 1262, 1264, 1266. Asdiscussed above, stratum refers to the number of intermediatesynchronous nodes between a base station 1204 and a global positioningsystem (GPS) server 1202. A base station 1204 a that is one hop awayfrom the global positioning system (GPS) server 1202 may have a stratumof Stratum-0 1262 a. Base stations 1204 b-c that are two hops away fromthe global positioning system (GPS) server 1202 may have a stratum ofStratum-1 1262 b-c. A base station 1204 d that is three hops away fromthe global positioning system (GPS) server 1202 may have a stratum ofStratum-2 1262 d.

Each base station 1204 may derive stratum based on the stratum of theimmediately preceding base station 1204 in the line to the globalpositioning system (GPS) server 1202. For example, a base station 1204 dwith a stratum of Stratum-2 1262 d may derive the stratum from the basestation 1204 b with a Stratum-1 1262 b. A home evolved NodeB (HeNB) 1260that is unsynchronized may derive the stratum from each base station1204 that the home evolved NodeB (HeNB) 1260 receives synchronizationinformation from. For example, the home evolved NodeB (HeNB) 1260 mayderive a Stratum-3 1264 based on the Stratum-2 1262 d of the basestation 1204 d. The home evolved NodeB (HeNB) 1260 may also derive aStratum-2 1266 based on the Stratum-1 1262 c of the base station 1204 c.The derived stratum 1264, 1266 may be one greater than the stratum 1262of the preceding base station 1204. The home evolved NodeB (HeNB) 1260may select the base station 1204 with the corresponding lowest stratum1262 as the synchronizing base station. Thus, the home evolved NodeB(HeNB) 1260 may select base station 1204 c as the synchronizing basestation and Stratum-2 1266 as the current stratum.

FIG. 13 illustrates certain components that may be included within abase station 1301. A base station may also be referred to as, and mayinclude some or all of the functionality of, an access point, abroadcast transmitter, a NodeB, an evolved NodeB, etc. The base station1301 includes a processor 1303. The processor 1303 may be a generalpurpose single- or multi-chip microprocessor (e.g., an ARM), a specialpurpose microprocessor (e.g., a digital signal processor (DSP)), amicrocontroller, a programmable gate array, etc. The processor 1303 maybe referred to as a central processing unit (CPU). Although just asingle processor 1303 is shown in the base station 1301 of FIG. 13, inan alternative configuration, a combination of processors (e.g., an ARMand DSP) could be used.

The base station 1301 also includes memory 1305. The memory 1305 may beany electronic component capable of storing electronic information. Thememory 1305 may be embodied as random access memory (RAM), read onlymemory (ROM), magnetic disk storage media, optical storage media, flashmemory devices in RAM, on-board memory included with the processor,EPROM memory, EEPROM memory, registers, and so forth, includingcombinations thereof.

Data 1307 and instructions 1309 may be stored in the memory 1305. Theinstructions 1309 may be executable by the processor 1303 to implementthe methods disclosed herein. Executing the instructions 1309 mayinvolve the use of the data 1307 that is stored in the memory 1305. Whenthe processor 1303 executes the instructions 1309, various portions ofthe instructions 1309 a may be loaded onto the processor 1303, andvarious pieces of data 1307 a may be loaded onto the processor 1303.

The base station 1301 may also include a transmitter 1311 and a receiver1313 to allow transmission and reception of signals to and from the basestation 1301. The transmitter 1311 and receiver 1313 may be collectivelyreferred to as a transceiver 1315. An antenna 1317 may be electricallycoupled to the transceiver 1315. The base station 1301 may also include(not shown) multiple transmitters, multiple receivers, multipletransceivers and/or additional antennas.

The various components of the base station 1301 may be coupled togetherby one or more buses, which may include a power bus, a control signalbus, a status signal bus, a data bus, etc. For the sake of clarity, thevarious buses are illustrated in FIG. 13 as a bus system 1319.

FIG. 14 illustrates certain components that may be included within awireless communication device 1414. The wireless communication device1414 may be an access terminal, a mobile station, a user equipment (UE),etc. The wireless communication device 1414 includes a processor 1403.The processor 1403 may be a general purpose single- or multi-chipmicroprocessor (e.g., an ARM), a special purpose microprocessor (e.g., adigital signal processor (DSP)), a microcontroller, a programmable gatearray, etc. The processor 1403 may be referred to as a centralprocessing unit (CPU). Although just a single processor 1403 is shown inthe wireless communication device 1414 of FIG. 14, in an alternativeconfiguration, a combination of processors (e.g., an ARM and DSP) couldbe used.

The wireless communication device 1414 also includes memory 1405. Thememory 1405 may be any electronic component capable of storingelectronic information. The memory 1405 may be embodied as random accessmemory (RAM), read only memory (ROM), magnetic disk storage media,optical storage media, flash memory devices in RAM, on-board memoryincluded with the processor, EPROM memory, EEPROM memory, registers, andso forth, including combinations thereof.

Data 1407 and instructions 1409 may be stored in the memory 1405. Theinstructions 1409 may be executable by the processor 1403 to implementthe methods disclosed herein. Executing the instructions 1409 mayinvolve the use of the data 1407 that is stored in the memory 1405. Whenthe processor 1403 executes the instructions 1409, various portions ofthe instructions 1409 a may be loaded onto the processor 1403, andvarious pieces of data 1407 a may be loaded onto the processor 1403.

The wireless communication device 1414 may also include a transmitter1411 and a receiver 1413 to allow transmission and reception of signalsto and from the wireless communication device 1414. The transmitter 1411and receiver 1413 may be collectively referred to as a transceiver 1415.A first antenna 1417 a and a second antenna 1417 b may be electricallycoupled to the transceiver 1415. The wireless communication device 1414may also include (not shown) multiple transmitters, multiple receivers,multiple transceivers and/or additional antennas.

The various components of the wireless communication device 1414 may becoupled together by one or more buses, which may include a power bus, acontrol signal bus, a status signal bus, a data bus, etc. For the sakeof clarity, the various buses are illustrated in FIG. 14 as a bus system1419.

The techniques described herein may be used for various communicationsystems, including communication systems that are based on an orthogonalmultiplexing scheme. Examples of such communication systems includeOrthogonal Frequency Division Multiple Access (OFDMA) systems,Single-Carrier Frequency Division Multiple Access (SC-FDMA) systems, andso forth. An OFDMA system utilizes orthogonal frequency divisionmultiplexing (OFDM), which is a modulation technique that partitions theoverall system bandwidth into multiple orthogonal sub-carriers. Thesesub-carriers may also be called tones, bins, etc. With OFDM, eachsub-carrier may be independently modulated with data. An SC-FDMA systemmay utilize interleaved FDMA (IFDMA) to transmit on sub-carriers thatare distributed across the system bandwidth, localized FDMA (LFDMA) totransmit on a block of adjacent sub-carriers, or enhanced FDMA (EFDMA)to transmit on multiple blocks of adjacent sub-carriers. In general,modulation symbols are sent in the frequency domain with OFDM and in thetime domain with SC-FDMA.

The term “determining” encompasses a wide variety of actions and,therefore, “determining” can include calculating, computing, processing,deriving, investigating, looking up (e.g., looking up in a table, adatabase or another data structure), ascertaining and the like. Also,“determining” can include receiving (e.g., receiving information),accessing (e.g., accessing data in a memory) and the like. Also,“determining” can include resolving, selecting, choosing, establishingand the like.

The phrase “based on” does not mean “based only on,” unless expresslyspecified otherwise. In other words, the phrase “based on” describesboth “based only on” and “based at least on.”

The term “processor” should be interpreted broadly to encompass ageneral purpose processor, a central processing unit (CPU), amicroprocessor, a digital signal processor (DSP), a controller, amicrocontroller, a state machine, and so forth. Under somecircumstances, a “processor” may refer to an application specificintegrated circuit (ASIC), a programmable logic device (PLD), a fieldprogrammable gate array (FPGA), etc. The term “processor” may refer to acombination of processing devices, e.g., a combination of a DSP and amicroprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The term “memory” should be interpreted broadly to encompass anyelectronic component capable of storing electronic information. The termmemory may refer to various types of processor-readable media such asrandom access memory (RAM), read-only memory (ROM), non-volatile randomaccess memory (NVRAM), programmable read-only memory (PROM), erasableprogrammable read only memory (EPROM), electrically erasable PROM(EEPROM), flash memory, magnetic or optical data storage, registers,etc. Memory is said to be in electronic communication with a processorif the processor can read information from and/or write information tothe memory. Memory that is integral to a processor is in electroniccommunication with the processor.

The terms “instructions” and “code” should be interpreted broadly toinclude any type of computer-readable statement(s). For example, theterms “instructions” and “code” may refer to one or more programs,routines, sub-routines, functions, procedures, etc. “Instructions” and“code” may comprise a single computer-readable statement or manycomputer-readable statements.

The functions described herein may be implemented in software orfirmware being executed by hardware. The functions may be stored as oneor more instructions on a computer-readable medium. The terms“computer-readable medium” or “computer-program product” refers to anytangible storage medium that can be accessed by a computer or aprocessor. By way of example, and not limitation, a computer-readablemedium may comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium that can be used to carry or store desired program code inthe form of instructions or data structures and that can be accessed bya computer. Disk and disc, as used herein, includes compact disc (CD),laser disc, optical disc, digital versatile disc (DVD), floppy disk andBlu-ray® disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of theclaims. In other words, unless a specific order of steps or actions isrequired for proper operation of the method that is being described, theorder and/or use of specific steps and/or actions may be modifiedwithout departing from the scope of the claims.

Further, it should be appreciated that modules and/or other appropriatemeans for performing the methods and techniques described herein, suchas those illustrated by FIGS. 2, 8, and 10-11, can be downloaded and/orotherwise obtained by a device. For example, a device may be coupled toa server to facilitate the transfer of means for performing the methodsdescribed herein. Alternatively, various methods described herein can beprovided via a storage means (e.g., random access memory (RAM), readonly memory (ROM), a physical storage medium such as a compact disc (CD)or floppy disk, etc.), such that a device may obtain the various methodsupon coupling or providing the storage means to the device.

It is to be understood that the claims are not limited to the preciseconfiguration and components illustrated above. Various modifications,changes and variations may be made in the arrangement, operation anddetails of the systems, methods, and apparatus described herein withoutdeparting from the scope of the claims.

What is claimed is:
 1. A base station configured for use in a wirelesscommunication network, the base station comprising: means for monitoringone or more synchronization signals; and means for determining ahierarchical level of the base station in a synchronization hierarchy ofbase stations of the wireless communication network.
 2. The base stationof claim 1, further comprising: means for acquiring synchronizationbased on the one or more synchronization signals; and wherein the meansfor determining is configured to determine the hierarchical level basedon the one or more synchronization signals used by the means foracquiring to acquire synchronization.
 3. The base station of claim 1,wherein the one or more synchronization signals are received from one ormore other respective base stations, and wherein the one or moresynchronization signals contain respective data to indicate one or morerespective hierarchical levels of the one or more respective basestations in the synchronization hierarchy.
 4. The base station of claim3, wherein the means for determining is configured to increment ahierarchical level indicated by the data to obtain a hierarchical levelof the base station.
 5. The base station of claim 1, wherein the basestation is configured to refrain from transmitting during at least onetime period, and wherein the means for monitoring is configured tomonitor the one or more synchronization signals during the at least onetime period.
 6. The base station of claim 5, wherein the at least onetime period corresponds to at least one time period during which one ormore other base stations of at least one level of the synchronizationhierarchy greater than a hierarchical level of the base station transmitthe respective one or more synchronization signals.
 7. The base stationof claim 1, further comprising: means for transmitting a synchronizationsignal during a time period associated with the hierarchical level ofthe base station.
 8. A method of synchronizing a base station in awireless network, the method comprising: monitoring, at the basestation, one or more synchronization signals; and determining, at thebase station, a hierarchical level of the base station in asynchronization hierarchy of base stations of the wireless communicationnetwork.
 9. The method of claim 8, further comprising: acquiringsynchronization based on the one or more synchronization signals; andwherein the determining comprises determining the hierarchical levelbased on the one or more synchronization signals used in the acquiringto acquire synchronization.
 10. The method of claim 8, wherein the oneor more synchronization signals are received from one or more otherrespective base stations, and wherein the one or more synchronizationsignals contain respective data to indicate one or more respectivehierarchical levels of the one or more respective base stations in thesynchronization hierarchy.
 11. The method of claim 10, wherein thedetermining includes incrementing a hierarchical level indicated by thedata to obtain a hierarchical level of the base station.
 12. The methodof claim 8, further comprising: refraining from transmitting during atleast one time period, and wherein the monitoring comprises monitoringthe one or more synchronization signals during the at least one timeperiod.
 13. The method of claim 12, wherein the at least one time periodcorresponds to at least one time period during which one or more otherbase stations of at least one level of the synchronization hierarchygreater than a hierarchical level of the base station transmit therespective one or more synchronization signals.
 14. The method of claim8, further comprising: transmitting a synchronization signal during atime period associated with the hierarchical level of the base station.15. A base station configured for use in a wireless communicationnetwork, comprising: a processor; and memory in electronic communicationwith the processor and containing instructions that are executable bythe processor to implement operations including: monitoring, at the basestation, one or more synchronization signals; and determining, at thebase station, a hierarchical level of the base station in asynchronization hierarchy of base stations of the wireless communicationnetwork.
 16. The base station of claim 15, wherein the operationsfurther include: acquiring synchronization based on the one or moresynchronization signals; and wherein the determining comprisesdetermining the hierarchical level based on the one or moresynchronization signals used in the acquiring to acquiresynchronization.
 17. The base station of claim 15, wherein the one ormore synchronization signals are received from one or more otherrespective base stations, and wherein the one or more synchronizationsignals contain respective data to indicate one or more respectivehierarchical levels of the one or more respective base stations in thesynchronization hierarchy.
 18. The base station of claim 17, wherein thedetermining includes incrementing a hierarchical level indicated by thedata to obtain a hierarchical level of the base station.
 19. The basestation of claim 15, wherein the operations further include: refrainingfrom transmitting during at least one time period, and wherein themonitoring comprises monitoring the one or more synchronization signalsduring the at least one time period.
 20. The base station of claim 19,wherein the at least one time period corresponds to at least one timeperiod during which one or more other base stations of at least onelevel of the synchronization hierarchy greater than a hierarchical levelof the base station transmit the respective one or more synchronizationsignals.
 21. The base station of claim 15, wherein the operationsfurther include: transmitting a synchronization signal during a timeperiod associated with the hierarchical level of the base station.
 22. Acomputer-program product comprising a non-transitory computer-readablemedium having instructions thereon that, if executed in a base stationof a wireless communication network, result in the implementation ofoperations comprising: monitoring, at the base station, one or moresynchronization signals; and determining, at the base station, ahierarchical level of the base station in a synchronization hierarchy ofbase stations of the wireless communication network.
 23. Thecomputer-program product of claim 22, wherein the operations furthercomprise: acquiring synchronization based on the one or moresynchronization signals; and wherein the determining comprisesdetermining the hierarchical level based on the one or moresynchronization signals used in the acquiring to acquiresynchronization.
 24. The computer-program product of claim 22, whereinthe one or more synchronization signals are received from one or moreother respective base stations, and wherein the one or moresynchronization signals contain respective data to indicate one or morerespective hierarchical levels of the one or more respective basestations in the synchronization hierarchy.
 25. The computer-programproduct of claim 24, wherein the determining includes incrementing ahierarchical level indicated by the data to obtain a hierarchical levelof the base station.
 26. The computer-program product of claim 22,wherein the operations further comprise: refraining from transmittingduring at least one time period, and wherein the monitoring comprisesmonitoring the one or more synchronization signals during the at leastone time period.
 27. The computer-program product of claim 26, whereinthe at least one time period corresponds to at least one time periodduring which one or more other base stations of at least one level ofthe synchronization hierarchy greater than a hierarchical level of thebase station transmit the respective one or more synchronizationsignals.
 28. The computer-program product of claim 22, wherein theoperations further comprise: transmitting a synchronization signalduring a time period associated with the hierarchical level of the basestation.